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Mud Losses in Fractured
Carbonate Formations
John Emeka Udegbunam, University
of Stavanger, Norway
Carbonate Reservoirs
• Sedimentary rocks made of carbonate minerals
– limestone, dolomite and chalk
• Hold > 60 % oil and 40 % gas reserves globally
• Middle East accounts for 70 % oil and 90 % gas
reserves [1]
• Most of these carbonates are naturally fractured
• Fracture pathways can be beneficial during HC
production
• Mud losses is common during drilling
Naturally Fractured Reservoirs
Figure 1: A naturally fractured reservoir (left) and its model (right) [3]
Naturally Fractured Reservoirs
• Have a continuous network of well-distributed
fractures
• Absence of fluid transition zones
• Constant PVT properties with depth
• Low producing GOR
• Low pressure drop around producing well
• Presence of fracture network gas cap [2]
Naturally Fractured Reservoirs
(a) Conventional reservoir (b) Fractured reservoir
Figure 2: Fluid transition zones in a non-fractured carbonate reservoir [2]. There is absence of transition zone in a naturally fractured reservoir due to negligible capillary effects.
Fracture Identification
• Significant mud loss while drilling
• Lab core examination
• Observation of outcrops
• Special behaviour in transient pressure analysis
• Use of televiewer (logging)
• Seismic evaluations [2]
Fracture Network
• A single fracture (defined by width, size and
nature)
• Fracture system: a set of parallel fractures
- can align horizontally or vertically
• Fracture network: two or more linked fracture
system [2]
Fracture Network
Figure 3: Natural Fracture network [4]
Group Fracture Parameters
Figure 4: Fracture systems
• Linear fracture density
- (LFDV/LFDX;
LFDH/LFDZ)
• Fracture intensity (FINT)
Wettability and Capillary Pressure
Figure 5: Wettability measurement for water-oil-solid system [2]
• Wetting phase
• Non-wetting phase
• Contact angle (θ)
• Capillary pressure
Pc = Pnw – Pw = Po – Pw
Wetting index (cosθ)
Contact angle
(θ)
Wetting condition
1.0 0° Completely water-wet
0 90° Neutral wet
-1.0 180° Completely oil-wet
Table 1: Wetting indices for different wetting conditions
Lost Circulation in Fractured Reservoirs
• High tendency for drilling fluid losses
• Causes kick, blowout
• Well control problem
• Invasive formation damage
Effects of Drilling Fluid Section and Reservoir Wettability
• Drilling fluid (mud) - OBM, WBM & SBM
• Water-wet reservoir vs WBM
• Oil-wet reservoir vs OBM
• The liquid phase will imbibe in the matrix
- additional mud loss
Measures to Reduce Lost Circulation
• Proper drilling fluid selection
• Proper fracture identification
• Use of LCMs
• Application of underbalanced operations
• Use of advanced drilling methods- MPD & DGD
Conclusion
• Mud engineers should consider wetting conditions of fractured carbonate reservoirs
References 1. Schlumberger, 2012: “Carbonate Reservoirs,” slb website.
2. Torsæter, O., 2012: “Reservoir Engineering of Naturally Fractured Reservoir”, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.
3. Warren, J. E., and Root, P. J., 1963: “The Behavior of Naturally Fractured Reservoirs”, SPE Journal of Petroleum Technology, 426-PA, September, 245-255.
4. Narr, W., Schechter, D. W., and Thompson, L. B., 2006: “Naturally Fractured Reservoir Characterization”, Text book Series, SPE, Richardson, Texas, USA.